60-year-old LeMond -- who is considered one of the greatest cyclists ever -- said in a statement he was concerned about his health after weeks of fatigue ... so he went to see his doctor.

"Following a series of tests and a bone marrow biopsy, which was completed last week, I received my formal diagnosis last Friday," the cyclist revealed.

LeMond said the diagnosis was chronic myelogenous leukemia, a form of cancer that is non-life threatening and treatable -- and he starts treatment this week.

"I should be feeling better in a few weeks and for the near future," LeMond said.

"My daily schedule will be altered only a little and I have been told that in a few months, I should be in remission."

LeMond added, "I will keep everyone updated about my health and treatments in the months ahead but for now, I believe I couldn't be in better hands."

LeMond says he may not be able to attend the Tour de France in July ... but he's trying to arrange a way he can be there for his friends.

"I will look forward to returning to the Tour next summer!"

LeMond started his career in 1981 and won three Tour de France and two Road Race Championships -- the first American cyclist to win that race.

He is the first pro bicyclist to sign a million-dollar contract ... as well as the first to make the Sports Illustrated cover.

LeMond retired in 1994 and was inducted into the U.S. Bicycling Hall of Fame in 1996.